1. Technical Field
This invention relates generally to power generation systems, and, more particularly, to a system and method for controlling load dump voltage of a permanent magnet alternator.
2. Description of the Related Art
Increasing electrical power demand in automotive vehicles has spurred investigation into high power, high efficiency power generation systems, inasmuch as conventional, so-called Lundell type alternators (claw-tooth pole, inductive type) have reached maximum capacity servicing present load demands. In response, it has been proposed in the automotive industry to adopt a high voltage power system to meet this increased power demand, as well as to reduce wiring harness cost and improve efficiency, among other things. In particular, a 42 volt standard has been proposed for both Europe and the United States. However, a problem arises with respect to load dump compliance for 42 volt alternators.
As background, a load dump condition is where the electrical load on the alternator, including the battery, goes instantly to zero, for example, via a broken or disconnected wire. A load dump test determines whether voltage transients output by the alternator under the conditions described above exceed a predetermined threshold. The predetermined threshold is selected to protect, for example, semiconductor devices that may draw power from the power system. Under the present standard, the load dump threshold is 60 volts.
In a conventional 42 volt Lundell type alternator, when the load dump condition occurs, avalanche diodes employed in a bridge portion thereof shorts the phase windings, thereby limiting the output voltage to the avalanche voltage limit of the diode. This limit is within the load dump threshold. However, high energy absorbing diodes are required, which may increase the cost of the system.
An attractive alternative for high power generation is a permanent magnet (PM) alternator, due to characteristics such as high power density, high efficiency and the like. In a conventional 42 volt PM alternator, the winding voltage output amplitude varies linearly with a rotational speed, and the average output voltage is controlled to 42 volts by means of, for example, a silicon controlled rectifier (SCR) bridge rectifier. However, the peak voltage, which is proportional to the speed, exceeds the present day 60 volt limit during a load dump condition.
Hoffman et al., U.S. Pat. No. 6,181,111, disclose an alternator for an automotive vehicle having a rotating permanent magnet (i.e., rotor) configured to induce a voltage in a stationary armature in which a three-phase winding is disposed. The winding is configured in a delta arrangement and is coupled to a bridge comprising six controlled rectifiers, an upper and lower rectifier for each phase. The rectifiers are disclosed as being a silicon controlled rectifiers (SCRs). Hoffman et al. further disclose that the alternator may be configured to produce a conventional output voltage of 14.5 volts. Hoffman et al. further disclose a relatively complex circuit for responding to a load dump condition. The complex circuit increases cost and may limit its usefulness in large scale production. Moreover, Hoffman et al. recognize that a high transient voltage may nonetheless exist for one alternation, notwithstanding the presence of the above complex circuit, and therefore provide for a metal oxide varistor (MOV) across the alternator output terminals as a limiting device. Applicants believe that such a device would have to be selected to dissipate a large amount of power and accordingly would be of increased cost, which is undesirable.
There is therefore a need for an improved power generation system that minimizes or eliminates one or more of the problems as set forth above.
One object of the present invention is to solve one or more of the problems set forth in the Background. A system in accordance with the present invention exhibits a reduced cost relative to conventional systems for power generation, and is effective in controlling load dump voltage transients so as to not exceed predetermined thresholds.
A power generation system according to the invention includes a permanent magnet (PM) alternator, a bridge, a controller, and a suppression device. The PM alternator includes a multi-phase stator winding that provides a full wave output. The bridge is coupled to the stator winding output and includes controllable rectifiers responsive to conduction control signals. The bridge further includes a rectified output configured to be coupled to a battery and various electrical loads. The controller is configured to generate the conduction control signals and is operable to control the bridge output to a first predetermined voltage corresponding to an operating voltage. In one embodiment, the operating voltage is nominally 42 volts. The suppression device is configured to suppress the conduction control signals when the bridge output exceeds a second predetermined voltage (e.g., 55 volts in one embodiment) greater than the first predetermined voltage. The second predetermined voltage is selected to be less than the load dump threshold limit.
In a preferred embodiment, the controllable rectifiers comprise silicon controlled rectifiers (SCR), the conduction control signals comprise gate pulses destined for the gate terminals of the SCRs, and the suppression device includes a divider network, a peak detector, and a comparator. The divider network is coupled to the bridge output for attenuation and for generating a scaled voltage. The peak detector is responsive to the scaled voltage and is coupled to a first input terminal of the comparator. The comparator has a reference voltage coupled to a second input terminal thereof, and has an output terminal producing a gate enable signal that changes logic state when the scaled voltage exceeds the reference voltage. The change in state causes the controller to discontinue generation of the gate pulse signals. A relatively low-power and therefor low cost varistor or zener diode is coupled across the bridge output.